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The control of water loss in trees

The cuticle is the primary barrier against uncontrolled foliar water loss, and is comprised of a continuous cutin membrane, waxes, and polysaccharides. The cuticle ultimately controls the transfer of water on both an intra-ceullular level and on an atmospheric level, thereby aiding with the reduction of water loss through the epidermis (Riederer & Schreiber, 2001; Watson, 2006). In conifers, water loss is controlled not only by very thick waxy cuticles, but by the leaf area being segmented into a massive abundance of smaller single leaf areas – this segmentation of the leaf ‘mass’ is known as a xerophytic adaption (Watson, 2006). Further, and as previously established, sun leaves will also be smaller in order to reduce surface area and, subsequently, transpirational loss (Givnish, 1988; Nobel, 1976).

The stomata, found below the cuticle layer, facilitate the rate of transpiration. They are usually less abundant on the upper epidermis than on the underside, as upper epidermis abundance would provide more risk of significant water loss by the leaf. Stomata begin to close when hydration levels of the soil begin to fall, thus ensuring that necessary water is retained within the plant. However, as stomata are required for gas exchange, their full closure halts any gaseous exchanges. To combat this however, the cuticle layer can provide for limited gas exchange so leaf operations can continue; albeit at a reduced level (Boyer et al., 1997).

Stomata control moisture loss through the two guard cells that surround and regulate each stomatal pore. To optimize the trade-off between carbon dioxide induction (which also occurs via the stomata) and transpirational water loss, stomata sense and respond to a range of environmental signals that include ambient carbon dioxide concentration and soil moisture levels. As moisture levels drop, stomata will reduce their size in response via the closure of the guard cells (Doheny-Adams et al., 2012). Abscisic acid will regulate such stomatal closure under drought conditions, and the flow of positively-charged potassium ions out of the guard cells will facilitate their closure by drawing out water alongside through osmotic processes (Karban, 2015).

In very drastic drought conditions, leaves may control water loss by abscising from the tree, thus reducing potential transpirational area and also reducing the overall water demand of the tree. Juglans ssp. for instance are known to shed leaves in times of severe drought, and such an act is defined as vulnerability segmentation (Tyree et al., 1993). Typically, leaves will be shed from the lower crown primarily, as their role is of lesser criticality than the upper crown’s leaves, and the increased competition for light in the lower canopy means retaining such leaves may be impractical (Achten et al., 2010). As a side note, buds will form within leaf axils as soon as leaves begin to develop. Therefore, if leaves are shed, new leaves can readily be grown again, and the same process will begin once more (Shigo, 1986). This adaption essentially means trees can continue to live following intentional or unintentional (herbivory, pruning, etc) defoliation – this is of direct benefit to the tactic of leaf shedding in drought conditions.

Additional means of controlling water loss include altering the angle at which the leaves face the sun (such as with Fraxinus excelsior), through the growth of small hairs that trap air and make it more difficult for water to escape in dry conditions or conversely aid with water release by keeping the leaf surface clear of moisture build-up during times of very high humidity, and by adopting one of two leaf macro-morphological adaptations: (1) grow rather thin leaves in times where conditions are adverse, shedding them once conditions improve, and / or (2) growing leaves that are small and covered with a leathery cuticle, thereby persisting through the adverse conditions and increasing photosynthetic rates once conditions improve again (Davis, 2015).